1. Field of the Invention
The present invention relates to a recording medium having a resin substrate.
2. Description of the Related Art
As a method of increasing the recording density of an optical disk, examinations have been made on increasing the numerical aperture (NA) of an objective lens and shortening the wavelength of a laser to be used. In a 3.5-inch magneto-optical disk drive being used at present, the numerical aperture of an objective lens is 0.55. By increasing the numerical aperture of the objective lens up to 0.70, the size of a beam spot can be reduced from about 1.0 μm as a conventional size to 0.78 μm. A similar effect can be obtained by shortening the wavelength of a laser to be used. By changing a laser having a wavelength of 650 nm being used at present into a blue-violet laser having a wavelength of 405 nm, the conventional beam spot size can be reduced from about 1.0 μm to 0.65 μm. As a result, the marks on the optical disk can be shortened and the track pitch can be narrowed to thereby allow high-density recording.
In the case of applying such means for allowing high-density recording, it is known that the margin to the warpage of a resin substrate due to its thermal deformation not particularly becoming a problem in the prior art is reduced. This is due to the fact that even if the warpage of a single-layer resin substrate is the same in level, optical aberration becomes likely to occur because of the increasing of the numerical aperture or the shortening of the wavelength, causing a perturbation of the shape of a beam spot and therefore largely interfering with stable recording and reproducing operations. Further, regarding the problem of optical aberration due to the increasing of the numerical aperture, the optical aberration is sensitive to variations in thickness of the conventional resin substrate having a thickness of 1.2 mm, causing an extremely large amount of optical aberration due to variations in thickness of the resin substrate.
Therefore, it is essential to reduce the thickness of the substrate in the case of using an objective lens having a high numerical aperture. However, when the thickness of the substrate is reduced, the warpage due to thermal deformation tends to become large, so that optical aberration due to the warpage instead becomes likely to occur. In a conventional 3.5-inch magneto-optical disk, a recording layer is formed on a first surface of a transparent resin substrate in the range of 18 to 42 mm along the radius of the substrate from the center thereof. This recording layer and a second surface of the resin substrate as an incident surface for a laser beam are coated with organic protective films.
Japanese Patent Laid-open No. Hei 11-16211 discloses a method of suppressing the warpage of a resin substrate by forming a thermal deformation suppressing film having a thermal expansion coefficient smaller than that of the resin substrate on the second surface of the resin substrate in the same radial range as that of the recording layer formed on the first surface of the resin substrate to thereby suppress the thermal deformation of the resin substrate. However, in the case of applying this method to a polycarbonate substrate being widely used for an optical disk, moisture is absorbed into the polycarbonate substrate from a position where the recording layer and the thermal deformation suppressing film are not formed.
The moisture once absorbed into the polycarbonate substrate is hard to expel to the atmosphere with the medium configuration that the substrate is closed by the recording layer and the thermal deformation suppressing film as moisture resistant materials. Accordingly, when a sudden rise in temperature occurs, the moisture is expelled to the boundary between the polycarbonate substrate and the recording layer. As a result, there is a possibility of corrosion of the recording layer. This problem is remarkable particularly in a substrate formed of polycarbonate resin or amorphous polyolefin resin having low birefringence and low saturated moisture absorbance as applied to an optical disk for high-density recording. The moisture absorbed or gas contained in such a resin is generated from the boundary between the recording layer and the resin substrate and the boundary between the thermal deformation suppressing film and the resin substrate by a sudden rise in ambient temperature. As a result, the recording layer and the thermal deformation suppressing film are separated from the resin substrate by the pressure of the moisture or gas generated, causing a semispherical dilation on the substrate, resulting in recording and reproducing errors. This phenomenon is due to the fact that the moisture absorbed in such a resin having low saturated moisture absorbance is hard to move therein, so that the moisture or gas is prone to be expelled to the nearest boundary.
There is another problem of loss in optical characteristics. That is, although the thermal deformation suppressing film is transparent, there is a power loss of the laser beam due to reflection or absorption because the thermal deformation suppressing film is formed on the incident surface for the laser beam. The power loss of the laser beam causes a reduction in the margin to high-speed recording and reproduction by high-speed rotation of the optical disk. To cope with the corrosion of the recording layer and the loss in optical characteristics, Japanese Patent Laid-open No. Hei 1-292639 discloses a medium configuration that a thermal deformation suppressing film is formed on the second surface of a resin substrate in the radial range from the innermost circumference of the substrate to a position corresponding to the inner periphery of a recording region of a recording layer formed on the first surface of the substrate in such a manner that the thermal deformation suppressing film does not overlap the recording region.
This medium configuration in the prior art will now be described more specifically with reference to FIG. 1. Reference numeral 2 generally denotes a recording medium. The recording medium 2 includes a transparent resin substrate 4 having first and second surfaces opposite to each other. A recording layer 6 is formed on the first surface of the resin substrate 4. The resin substrate 4 has a center hole 5. A thermal deformation suppressing film 8 is formed on the second surface of the resin substrate 4 in the radial range from the center hole 5 to a position corresponding to the inner periphery of a recording region of the recording layer 6. Accordingly, the thermal deformation suppressing film 8 does not overlap the recording region of the recording layer 6. With this configuration that the thermal deformation suppressing film 8 is formed radially inside of the recording region, the resin substrate 4 is not closed by the recording layer 6 and the thermal deformation suppressing film 8 at the recording region, thereby preventing the corrosion of the recording layer 6. Furthermore, since the thermal deformation suppressing film 8 is formed outside the incident region for a laser beam, there is no problem of optical loss.
However, in this configuration that the thermal deformation suppressing film 8 is formed so as to radially range from the center hole 5 to the inner periphery of the recording region of the recording layer 6, the thermal deformation suppressing film 8 comes into contact with a reference plane of a spindle 12 in an optical disk drive as shown by an arrow 14, causing separation of the thermal deformation suppressing film 8 from the substrate 4. As a result, dust is prone to generate and the flatness of a contact portion between the recording medium 2 and the reference plane of the spindle 12 is lost to result in difficulty of stable recording and reproducing operations. This problem of separation of the thermal deformation suppressing film 8 may be solved by coating an organic protective film so as to cover the thermal deformation suppressing film 8.
However, the contact portion between the thermal deformation suppressing film 8 and the reference plane of the spindle 12 is a ring-shaped region at the innermost circumference of the substrate, so that a high peripheral speed of the substrate cannot be obtained in using a rotary applicator for coating the organic protective film, and it is therefore difficult to coat the organic protective film with a uniform thickness. Further, there is a problem in durability of the thermal deformation suppressing film in the case that it is not coated with the organic protective film. For example, daily cleaning of the laser beam incident surface of the optical disk by a user is carried out by using a cleaning agent containing alcohol and a wiping cloth or soft paper in many cases.
If the adhesion of the thermal deformation suppressing film to the resin substrate is low, separation of the thermal deformation suppressing film by the work of wiping off the cleaning agent is prone to occur. To verify this fact, the adhesion of the thermal deformation suppressing film was evaluated by a wipe test. More specifically, an SiN film having a thickness of 165 nm as the thermal deformation suppressing film was formed on a polycarbonate substrate by sputtering to evaluate the adhesion of the SiN film by the wipe test. The wipe test was made by wiping the SiN film with a swab impregnated with ethanol until the SiN film was separated off. In this wipe test, a load of 100 g was applied to the swab and wiping speed was set to 500 mm/min. Thus, the above-mentioned disk cleaning work is replicated. As the result of this wipe test, the SiN film was separated off by five reciprocating stokes of the swab. This result shows the necessity of the organic protective film for the thermal deformation suppressing film.
There is another problem in coating the organic protective film. In forming the thermal deformation suppressing film, the deposition of projecting foreign matter in or on the thermal deformation suppressing film is unavoidable. In a conventional coating method for the organic protective film, a line defect due to the projecting foreign matter appears on the organic protective film. The line defect extends from the projecting foreign matter toward the outer circumference of the organic protective film.